For the fish stock, put all the ingredients except the fish bones into a large pan, bring to the boil, then turn down the heat and simmer gently for 20 minutes.
Add the fish bones (or fish fillet) and bring back up to a simmer, skimming off any scum as it rises to the surface. Simmer for a further 20 minutes. Strain through a sieve into a clean pan and simmer a little longer, if necessary, until reduced to about 1.2 litres/2 pints. If not using immediately, cool and refrigerate or freeze for later use.
For the tomato sauce, heat the olive oil and garlic in a saucepan. As soon as the garlic starts to sizzle, add the tomatoes and simmer for 15-20 minutes, breaking them up as they cook. Cook until the sauce has reduced and thickened, then season with salt and pepper. The sauce is ready to use. If not using immediately, leave to cool, then refrigerate for up to a week or freeze for later use.
Season the John Dory fillets with some salt and pepper and lightly dust with flour. Heat the olive oil in a frying pan, add the fillets skin-side down and fry for 2-3 minutes until lightly browned. Season with a squeeze of lemon juice, turn the fish over and cook for a further minute. Remove from the pan and keep warm.
Add the Worcestershire sauce, gherkins, whole and chopped capers, tomatoes, tomato sauce and fish stock to the pan, bring to the boil then reduce a little to create the sauce. Add the fish carefully back to the pan for a minute to warm through in the sauce.
Serve immediately, drizzled with a little more olive oil and sprinkled with fresh parsley.